1. Field of the Invention
This invention relates to a fail-safe device for a heating appliance having a microcomputer-aided control, and more particularly provides a highly safe heating appliance which prevents malfunction of a control device originating from faulty operation of a microcomputer through the use of a heating start switch operating mechanically independently of control signals introduced via a keyboard.
2. Description of the Prior Art
In recent years, high frequency heating appliances have been put on the market as one of cooking tools with an electronic control system including in combination a microcomputer and a keyboard. In other words, the use of the microcomputer in high frequency heating appliances has offered numerous advantages: for example, timekeeping faculty, a plurality of selectable output levels, a plurality of selectable programmed cooking modes, availability of one or more sensors, etc. and therefore a remarkable improvement in convenience for the users and cooking applications. It is however known that the electronic microcomputer-aided control system is susceptible to noise induced from lightning, noise originating from a power supply and the high frequency heating appliance sometimes fails to operate due to such noise. To assure further effective utilization of the microcomputer, it is necessary for the control system to show a high resistance to noise or the like.
FIG. 1 is a perspective view illustrating the appearance of a conventional cooking appliance. In FIG. 1, a door 2 is openably and closably installed at the front of a main body 1. An operational panel 3 carries a display 4 and a keyboard 5, with the latter including heating mode selection keys 6, digit keys 7, a cooking start key 8, etc.
FIG. 2 is a circuit diagram of the conventional cooking appliance, wherein a power supply source 9 is connected to a high voltage transformer 14 through fuse 10, a switch 11 operatively interlocked with opening and closing movement of the door 2, a switch 12 and a relay contact 13. A high frequency oscillator is conventionally made of a magnetron 15 for which a power supply circuit is provided by a half-wave rectifying voltage multiplier including a capacitor 16, a diode 17 and the high voltage transformer 14.
A control board 18 is supplied with power from a low voltage transformer 19, which board carries a microcomputer 20 connected to a keyboard 5 via an interface 21. The interface 21 converts signals from the keyboad 5 into signals which are compatible with the microcomputer 20. The microcomputer 20 permits the resultant signals to be displayed on a display panel 23 and to be processed pursuant to a stored program and provides its output signal for a relay coil 24 via a converter 22 for placing its relay contact 13 into closed or open position.
For example, when a desired cooking period is preset via the digit key or keys 7 on the keyboard 5 and the cooking start key 8 is actuated, the output signal of the microcomputer 20 energizes the relay coil 24 to force the relay contact 13 into closed position for a given period of time so that the magnetron 15 is supplied with an enabling voltage to commence high frequency heating. In the event that the door 2 is opened during the course of heating, both the switches 11 and 12 are opened so as to interrupt the oscillation.
Should a high voltage originating from lightning, for example, be impressed on the high frequency heating appliance with the above illustrated conventional circuit arrangement, such electronic components as the microcomputer 20, the interface 21 and the converter 22 may be destroyed with a possibility of developing from the converter 22 a signal which may energize the relay coil 24. In this case, the magnetron 15 starts oscillating as long as the door is in closed position.
Because of no limited heating period of time being preset, the appliance continues oscillating and remains in abnormal heating condition without a load and thus is in danger of being operated with a lack of security. In particular, a high voltage resulting from induced lightning would increase the possibility of destroying a low breakdown voltage circuit and the appliance can be regarded as being constantly exposed to a dangerous situation because lightning can occur any time in the daytime or at night.